Don't generate type IDs for formal generic instances

[HertzDevil]

The compiler traverses through every instantiation of a generic class type when it generates unique IDs for every type's metaclass for the Object#class primitive. This includes formal types:

class A(T)
end

class B(T) < A(T) # second `T` is an unbound type parameter referring to the first `T`
  @x : A(T) # unbound type parameter referring to the `T` of `B(T)`
end

# the generated `~metaclass` function references `A(T)` and `A(T).class` where
# `T` refers to the type parameter of the generic `B(T)`, see below

This PR skips those types because they can never be encountered in any real program. This reduces the number of unique types in a blank source file by about 21%; see here for a comparison.

[HertzDevil]

The ~metaclass LLVM function, which corresponds to Object#class, is implemented by a jump table. Compiled to x86-64 assembly, it looks like this:

.LBB1430_589:
        movl    $822, %eax
        retq

.LBB1430_590:
        movl    $590, %eax
        retq

.LBB1430_591:
        movl    $595, %eax
        retq

.LBB1430_592:
        movl    $598, %eax
        retq

.LBB1430_750:
        movl    $1498, %eax
        retq

.LBB1430_751:
; no code follows

.LJTI1430_0:
        .long   .LBB1430_1-.LJTI1430_0
        .long   .LBB1430_2-.LJTI1430_0
        .long   .LBB1430_3-.LJTI1430_0
; ...
        .long   .LBB1430_589-.LJTI1430_0 ; Value -> Value.class
        .long   .LBB1430_590-.LJTI1430_0 ; Object -> Object.class
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_591-.LJTI1430_0 ; Array(T) -> Array(T).class, T is some type parameter
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_592-.LJTI1430_0 ; Array(T) -> Array(T).class, T is a different type parameter
; ...
        .long   .LBB1430_751-.LJTI1430_0 ; unreachable!
        .long   .LBB1430_750-.LJTI1430_0 ; Proc(Hash(T, Nil), T, Nil) -> Proc(Hash(T, Nil), T, Nil).class
; the table does not actually end with `.LBB1430_751` because it is used only to
; fill in the gaps between the `crystal_instance_type_id`s of various unbound types

"~metaclass":
        movl    %edi, %eax
        movl    %eax, %ecx
        movq    %rcx, -8(%rsp)
        leaq    .LJTI1430_0(%rip), %rax
        movq    -8(%rsp), %rcx
        movslq  (%rax,%rcx,4), %rdx
        addq    %rax, %rdx
        jmpq    *%rdx

A blank source file emits 908 of extra lines in this table (everything after .LBB1430_590), plus code for 160 branch targets. They are all gone, so a fully filtered blank source on Compiler Explorer now contains only ~3.2k lines after this PR.